This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present technology. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present technology. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Coat deposits may hold significant amounts of hydrocarbon gases, such as methane, ethane, and propane, generally adsorbed onto the surface of the coal. A significant amount of natural gas reserves exist as adsorbed species within coalbeds. The natural gas from coalbeds, commonly referred to as “coalbed methane” (CBM), currently constitutes a major source of the natural gas production in the United States. The CBM is generally produced by depressurization of coal seams.
However, even using well stimulation methods, such as cavitation (see, for example, U.S. Pat. No. 5,147,111), only a small fraction of the CBM is economically recoverable. More specifically, depressurization is limited to higher permeability coalbeds. This is because as pressure is decreased, coal cleats (i.e., natural fractures) may collapse and decrease the permeability of the coalbed. Loss of permeability is particularly a concern for deep coalbeds, which may have a low initial permeability. Depressurization may also result in production of low-pressure gas needing significant power for compression to permit pipelining to market.
As an alternative to, or in conjunction with, depressurization, improved recovery of CBM may be obtained by injecting another gas into the coalbed. For example, CO2 may be used to enhance the production of CBM (see, for example, U.S. Pat. Nos. 4,043,395; 5,085,274; and 5,332,036). CO2 more strongly adsorbs to the coal than CBM and, thus, may displace adsorbed CBM. In other applications, nitrogen (N2), which less strongly adsorbs onto coal than CBM, may be used (see, for example, U.S. Pat. Nos. 5,014,785; 5,566,756; Scott R. Reeves, “Geological Sequestration of CO2 in Deep, Unmineable Coalbeds: An Integrated Research and Commercial-Scale. Field Demonstration Project,” SPE 71749 (Society of Petroleum Engineers, 2001); and Jichun. Zhu, et al., “Recovery of Coalbed Methane by Gas injection,” SPE 75255 (Society of Petroleum Engineers, 2002). N2, and other less strongly adsorbing gases, lower the partial pressure of the CBM components in the bulk gas phase, which causes the CBM to desorb from the coal. Both of these methods can maintain the coalbed at relatively high pressures and hence aid permeability by keeping the cleat system open.
Other gases have also been described as enhancing production of coalbed methane or modifying coalbeds for other purpose's. For example, U.S. Patent Publication No. 2007/0144747 describes a process for pretreating an underground coal bed to enhance the potential for carbon dioxide sequestration. The method involves injecting hydrogen into an underground coal bed, wherein the hydrogen is at a temperature below about 800° C.; extracting hydrogen and methane from the coalbed; separating the hydrogen and methane; delivering the methane as a product of the process; and injecting the separated hydrogen into the deposit to continue the process. When the sequestration of carbon dioxide is desired, hydrogen may be optionally produced from methane and carbon dioxide may optionally be injected for sequestration.
The methods above are generally limited by the availability of the gas in sufficient amounts for injection. Further, the cost of separation to isolate gases from either the produced gases or the atmosphere (for example, by a swing adsorption process) may be prohibitively expensive. After separation, the gases may need substantial compression (e.g., 2500 psia or more depending on subsurface depth) for injection into a formation. Thus, technology for improved recovery of coalbed methane would be valuable.
Other related material may be found in at least U.S. Patent Publication No. 2005/0201929, U.S. Pat. Nos. 5,402,847; 6,412,559; and 7,491,250, and P. van Hemert, et al., “Adsorption of carbon dioxide and a hydrogen-carbon dioxide mixture,” 2006 International Coalbed Methane Symposium (Tuscaloosa, Ala., May 22-26, 2006), Paper 0615.